<h1>A heading at peace with itself</h1>
In short, your HTML should do everything it can to convey the meaning, or semantics of the content in the page, while steering clear of describing how it should look. Web standards geeks call HTML code that does this semantic markup.
Writing semantic markup allows your HTML files to stand on their own as meaningful documents. People who, for whatever reason, cannot read these documents by viewing them in a typical desktop web browser will be better able to make sense of them this way. Visually impaired users, for example, will be able to use assistive software like screen readers to listen to the page as it’s read aloud, and the more clearly your HTML code describes the content’s meaning, the more sense tools like these will be able to make of it.
Best of all, however, semantic markup lets you apply new styles (presentation) and interactive features (behavior) without having to make many (or, in some cases, any!) changes to your HTML code.
From Sitepoint
